Uniflow scavenging is one of the generally known types of scavenging in two-stroke engines. In uniflow scavenging, gas flows in only one direction in the cylinder. Outlet and inlet ports are at opposite ends of the cylinder chamber. The fresh gas always pushes the exhaust gas in the same direction from the inlet to the outlet. Generally, inlet slots in the bottom dead center and one or more outlet valves in the top dead center are used for the gas exchange. A scavenging pump introduces pressurized fresh air through the inlet slots into the cylinder. When the outlet valve(s) is/are open, the exhaust gas from the cylinder has been previously driven out through the pressure gradient.
An internal combustion engine having variable cycles is known from DE 690 52 10 865, which operates alternately in the two-stroke process and in the four-stroke process, depending on the rotation speed and load of the engine. The internal combustion engine comprises a cylinder which is provided with a first inlet port and an outlet port which are defined at the upper region of the cylinder and with a second inlet port which is defined at the lower region of the cylinder; a cylindrical cylinder liner which is arranged in the cylinder and is provided with a third inlet port which is defined in the lower region of the cylinder liner; a sleeve which is arranged rotatable about the cylinder liner to selectively open and close the third inlet port, wherein the sleeve is provided with an integrally formed permanent magnet; rotation means for rotating the sleeve under the influence of electromagnetic forces which act on the permanent magnet; inlet port opening and closing means for selectively opening and closing the first inlet port at the upper region of the cylinder; outlet port opening and closing means for selectively opening and closing the exhaust gas duct in the upper region of the cylinder; over-comp action means for providing pressurized air to the first inlet port and to the second inlet port, and cycle mode selecting means for, on the one hand, setting the rotation means into motion to rotate the sleeve to open the third inlet port in coordination with the second inlet port, and to operate the exhaust gas duct opening and closing means, and on the other hand, to set the rotation means into motion to rotate the sleeve to close the third inlet port and to operate the inlet port and outlet port opening and closing means to operate the engine in four-stroke mode.
This means that when the internal combustion engine is operated in four-stroke mode, the gas exchange is controlled via the outlet port and inlet port opening and closing means arranged in the upper region of the cylinder. When the internal combustion engine is operated in two-stroke mode, the gas exchange takes place via the outlet opening and closing means arranged in the upper region of the cylinder and via the inlet ports arranged in the lower region in the cylinder, which in this case are released by turning the sleeve. The inlet ports in the upper region of the cylinder remain closed.
In the two-cycle mode, the gas exchange takes place as a pure uniflow scavenging. The sleeve does not control the actual gas exchange here. The opening and closing of the inlet ports takes place through the piston upper edge.
A control for a two-stroke internal combustion engine with uniflow scavenging is known from DE 865 237 B, which is achieved by providing outlet bores in the bottom of the working cylinder, which outlet bores alternately release and block openings of an exhaust pipe plate during the rotational movement of the working cylinder, and by providing inlet slots at the inner end of the cylinder, which inlet slots alternately release and re-block openings of an inlet ring enclosing the cylinder, which inlet ring is adjustably arranged on the crankcase for the purpose of regulating the inlet timing.
The charge cycle is performed by the rotating cylinder, by which the outlet bores and inlet slots are released and blocked. The inlet ring is adjustably arranged on the crankcase to regulate the inlet timing.
DE 197 00 412 A1 describes a two-stroke diesel engine with uniflow scavenging, with inlet slider and outlet valves controlled by the working piston. The inlet slider is mounted in the bore of the cylinder and the cylinder cover, with its conical surface of at least three tie rods, which are mounted in the cylinder surface, is pressed with springs in the axial direction of the cylinder against conical surfaces of the cylinder, and when displacing the piston just before reaching the bottom dead center, is entrained by this, so that an annular air gap is formed, and at the same time the three outlet valves in the cylinder head open from the three tie rods over the spring carrier. The cylinder bore and inlet slider bore have the same diameter. During the downward movement of the piston (working stroke), the piston ring surface hits against the inner ring surface of the inlet slider after about 9/10 of the piston stroke and opens the inlet cross-section. The spring carrier with the springs and the spring sleeve are thereby tensioned. During upward movement of the piston, the inlet slider is pulled over the tie rods by means of the tensioned springs with its conical surface against the conical surface of the cylinder and the inlet cross-section is closed.
Two-stroke engines having uniflow scavenging, having scavenging ports or charging slots in the cylinder wall and exhaust valves in the cylinder lids, are well known. They are usually designed as slow-running large engines and have good efficiencies. However, they have a significant disadvantage due to their long piston and cylinder and therefore have complex dimensions and large weights.
In the medium-speed two-stroke engines, pistons with a very long piston skirt, on the lower edge of which one or two piston rings are arranged, are used to prevent the scavenging air from being able to penetrate freely into the crankcase space. This also has an adverse effect with respect to the installation size of the engine and on the design effort.